1. Field of the Invention
The present invention relates to an image-reading apparatus for reading a character or graphics image on a document, and more particularly to an image-reading apparatus which supplies image data to an image processor connected to a computer or the like, or which is incorporated in a facsimile machine.
2. Description of the Related Art
In general, an image-reading apparatus for reading a character or graphics image comprises: a document table on which a document is mounted; and a carriage device movable in parallel to the document table and elongated in the first direction, i.e., the main scanning direction, in which the image portion within a given elongate region on the document is read. The carriage device contains the following structural components: an illumination device for illuminating a document; a focusing device for focusing the light beam reflected by the document on a predetermined position, i.e., the reflected light beam representing the image portion within the elongate region; and a conversion device, i.e., a CCD line sensor, for converting the reflected light beam focused by the focusing device into an electric image signal. These structural components are integrally assembled as one unit. The image-reading apparatus is provided with a driving device for shifting either the document table or the carriage device in a second direction, i.e., an auxiliary scanning direction perpendicular to the main scanning direction, so as to successively read the image portions on the document through the elongate region.
The image-reading apparatus reads an image as follows. First of all, a document is placed on the document table, with the image-shown side facing the carriage device. Then, the document is illuminated with the light-rays emitted from the illumination device of the carriage device. The light beam reflected by the document, i.e., information representing the character or graphics image shown on the document, is focused by the focusing device of the carriage device such that the reflected light beam is focused on the light-receiving surface of the CCD line sensor. Thus, the reflected light beam is converted into an electric image signal. In this manner, the image portion within the given elongate region is read in the main scanning direction. Next, the carriage device is shifted in parallel to the document table; alternatively, the document table is shifted in parallel to the carriage device. In this condition, the image portion within the elongate region is read in a similar manner to that mentioned above. With the above operations repeated, all character or graphics image portions shown on the document are sequentially read from the document and are converted into electric image signals. The electric image signals produced by the CCD line sensor are supplied to a memory or a image processing device as image information signals, so as to enable the image to be reproduced or processed.
Incidentally, it is desirable for an image-reading apparatus of the above-mentioned type to have a function of varying the reading density in accordance with purposes.
U.S. Pat. No. 4,571,061 discloses an image forming apparatus, which provided continuously variable copy ratio and which can thus copy an original image at any copy ratio desired.
A user operates the input console of this image forming apparatus, thereby inputting the copy ratio at which the original image is to be, for example, reduced and copied on a copy-paper sheet smaller than the original sheet. The copy ratio is changed to any desired value in accordance with the position of the lens located between the original and the photosensitive drum of the apparatus, more precisely, the ratio of the distance A between the original and the lens to the distance B between the lens and the drum. The copy ratio is given as: EQU 1/A+1/B=1/f
where f is the focal length of the lens.
The lens is shifted in accordance with the desired copy ratio input by operating the input console. Further, two drive devices drive the photosensitive drum and the original-scanning carriage device, incorporated in the apparatus, respectively, at the velocities determined by the copy ratio thus input.
In the image-reading apparatus using the CCD line sensor, the light beam accumulation time, which is the minimum time required for reliably reading the information supplied to the sensor, is maintained at a constant value. Thus, the reading width per line by which the CCD line sensor can read an image during each reading operation has to be equal to the distance for which the CCD line sensor is shifted in the auxiliary scanning direction during the light beam accumulation time. Since the number of pulses which are supplied to the pulse step motor is an integer, the pulse step motor shifts the CCD line sensor in the auxiliary scanning direction for a distance by the integer times. Therefore, in order to equalize the reading width per line with the distance for which the CCD line sensor is shifted in the auxiliary scanning direction, a transmission mechanism is interposed between the pulse step motor and the CCD line sensor. In the case where a plurality of linear reading densities are used, however, the distance for which the CCD line sensor is shifted in accordance with each of the linear densities may not correspond to the number of pulses which ar supplied to the pulse step motor through the transmission mechanism.